Aqueous-phase reforming of crude glycerol: effect of impurities on hydrogen production
The aqueous-phase reforming (APR) of a crude glycerol that originates from an industrial process and the effect of the individual components of crude glycerol on APR activity have been studied over 1 wt% Pt/Mg(Al)O, 1 wt% Pt/Al2O3, 5 wt% Pt/Al2O3 and 5 wt% Pt/C catalysts at 29 bar and 225 °C. The use of a 10 wt% alkaline crude glycerol solution in water, containing 6.85 wt% glycerol, 1.62 wt% soaps, 1.55 wt% methanol, and 0.07 wt% ester, resulted in a dramatic drop in APR activity compared to the corresponding 6.85 wt% solution of pure glycerol in water. Catalytic performance in crude glycerol APR increased in the order: 1 wt% Pt/Al2O3 < 5 wt% Pt/Al2O3 < 1 wt% Pt/Mg(Al)O < 5 wt% Pt/C. A H2 selectivity of only 1% was obtained with crude glycerol over a 1 wt% Pt/Al2O3 catalyst, while the same catalyst material under identical reaction conditions gave 64% H2 with pure glycerol. The cause of deactivation was investigated with synthetic mixtures that mimicked the composition of the crude glycerol and contained glycerol, methanol and varying amounts of NaCl and sodium oleate. The results showed that Na salts of fatty acids have a much more pronounced negative influence on APR activity than NaCl does and greatly inhibit H2 formation. Stearic acid and long chain aliphatics and olefins were shown to be formed and to be involved in the deactivation of the catalyst. The relatively high activity/selectivity of the 1 wt% Pt/Mg(Al)O could be attributed to intercalation of oleate/stearate in the sheets of the layered double hydroxide that is formed under reaction conditions.
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